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Lukas Lang
Lukas Lang
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While writing this question, I've come up with the following workaround:

r4 = x_ /. p : Verbatim[Pattern][a_, _] :>
       ((p :> Evaluate@HoldComplete[1 -> a]) /. HoldComplete -> Association)
(* x_ :> Association[1 -> x] *)

f /. r4
(* <|1 -> f|> *)

So as long as RuleDelayed does not see Association, everything is fine.

Update

@WReach pointed me to a clearer and more in-depth explanation of what's happening here. I think that answer covers everything from my questions.

My attempt at an explanation

It seems like as soon as the Association[...] expression is evaluated, it is replaced by an "atomic" object, that no longer allows RuleDelayed to "see" the values. (From here, we know that Assocations have the attribute HoldAllComplete, but it seems it's more severe than that)

The confusion now arises since this "atomic object" is always displayed in it's InputForm (well, short of the Association[...] vs. <|...|> thing), and reentering this form consequently fixes the issue, as the HoldRest attribute of RuleDelayed prevents a new object from being created at this point.

The only point I can't answer at all is "how to tell the difference between the two outputs", and I'm hoping someone else can provide a solution to this.

While writing this question, I've come up with the following workaround:

r4 = x_ /. p : Verbatim[Pattern][a_, _] :>
       ((p :> Evaluate@HoldComplete[1 -> a]) /. HoldComplete -> Association)
(* x_ :> Association[1 -> x] *)

f /. r4
(* <|1 -> f|> *)

So as long as RuleDelayed does not see Association, everything is fine.

My attempt at an explanation

It seems like as soon as the Association[...] expression is evaluated, it is replaced by an "atomic" object, that no longer allows RuleDelayed to "see" the values. (From here, we know that Assocations have the attribute HoldAllComplete, but it seems it's more severe than that)

The confusion now arises since this "atomic object" is always displayed in it's InputForm (well, short of the Association[...] vs. <|...|> thing), and reentering this form consequently fixes the issue, as the HoldRest attribute of RuleDelayed prevents a new object from being created at this point.

The only point I can't answer at all is "how to tell the difference between the two outputs", and I'm hoping someone else can provide a solution to this.

While writing this question, I've come up with the following workaround:

r4 = x_ /. p : Verbatim[Pattern][a_, _] :>
       ((p :> Evaluate@HoldComplete[1 -> a]) /. HoldComplete -> Association)
(* x_ :> Association[1 -> x] *)

f /. r4
(* <|1 -> f|> *)

So as long as RuleDelayed does not see Association, everything is fine.

Update

@WReach pointed me to a clearer and more in-depth explanation of what's happening here. I think that answer covers everything from my questions.

My attempt at an explanation

It seems like as soon as the Association[...] expression is evaluated, it is replaced by an "atomic" object, that no longer allows RuleDelayed to "see" the values. (From here, we know that Assocations have the attribute HoldAllComplete, but it seems it's more severe than that)

The confusion now arises since this "atomic object" is always displayed in it's InputForm (well, short of the Association[...] vs. <|...|> thing), and reentering this form consequently fixes the issue, as the HoldRest attribute of RuleDelayed prevents a new object from being created at this point.

The only point I can't answer at all is "how to tell the difference between the two outputs", and I'm hoping someone else can provide a solution to this.

Source Link
Lukas Lang
Lukas Lang
  • 34.4k
  • 1
  • 56
  • 99

While writing this question, I've come up with the following workaround:

r4 = x_ /. p : Verbatim[Pattern][a_, _] :>
       ((p :> Evaluate@HoldComplete[1 -> a]) /. HoldComplete -> Association)
(* x_ :> Association[1 -> x] *)

f /. r4
(* <|1 -> f|> *)

So as long as RuleDelayed does not see Association, everything is fine.

My attempt at an explanation

It seems like as soon as the Association[...] expression is evaluated, it is replaced by an "atomic" object, that no longer allows RuleDelayed to "see" the values. (From here, we know that Assocations have the attribute HoldAllComplete, but it seems it's more severe than that)

The confusion now arises since this "atomic object" is always displayed in it's InputForm (well, short of the Association[...] vs. <|...|> thing), and reentering this form consequently fixes the issue, as the HoldRest attribute of RuleDelayed prevents a new object from being created at this point.

The only point I can't answer at all is "how to tell the difference between the two outputs", and I'm hoping someone else can provide a solution to this.